1. Field of the Invention
The present invention relates to a method for manufacturing a nitride-based semiconductor device, and, more particularly, to a method for manufacturing a GaN-based semiconductor layer on a heterogeneous substrate.
2. Description of the Related Art
As group III nitride light emitting diodes (LED) employing an AlxInyGa(1−x−y)N-based (where 0≦x≦1, 0≦y≦1, 0≦x+y≦1; which will be referred to as “GaN-based” hereinafter) semiconductor have been developed, GaN semiconductors have been spotlighted as a light source of white, green, or blue light, and many advances have been made in related technology. However, the most important problem in terms of growth of GaN-based semiconductors is that a substrate, upon which the GaN-based semiconductor is grown, has yet to be remarkably enhanced in quality.
In order to solve the problems of the substrate for growing the GaN-based semiconductor, various attempts have been made, including usage of a silicon substrate as a substitute substrate. Conventionally, the GaN-based material has been grown on a sapphire substrate by MOCVD (metal organic chemical vapor deposition). However, due to lattice mismatch and thermal expansion coefficient differences between the sapphire substrate and the GaN-based semiconductor, the GaN-based semiconductor grown on the sapphire substrate has a negative crystallinity, causing lots of defects in the sapphire substrate. Additionally, the sapphire substrate is not only relatively expensive, but also inappropriate for large-scale devices.
On the contrary, silicon (Si) substrates are appropriate for the large-scale devices in addition to being inexpensive, and allow integration of other Si electronic devices or optical devices thereon. Due to these advantages, the Si substrate has been spotlighted as a substitute for the sapphire substrate. However, as with the sapphire substrate, due to lattice mismatch and thermal expansion coefficient differences between the Si substrate and the GaN-based semiconductor, it is difficult to grow a GaN-based semiconductor having good quality on the Si substrate. In order to solve this problem, attempts have been made to grow buffer layers formed from a variety of materials. Especially, various investigations into growing AlN or GaN buffer layers have been conducted, although satisfactory results have yet to be obtained. U.S. patent application Ser. No. 09/900,833 discloses a method for growing a GaN single crystal by an HVPE (Hydride Vapor Phase Epitaxy) growth process.
FIG. 1 is a side sectional view illustrating a nitride-based semiconductor device 10 manufactured according to a conventional method. Referring to FIG. 1, a low temperature AlN buffer layer 13 is grown on a Si substrate 11 having a primary plane of (111) at 500° C. to 700° C. by MOCVD. The semiconductor device 10 further comprises a desired GaN semiconductor layer 19 on the AlN buffer layer 13. The low temperature AlN buffer layer 13 serves to relieve stress caused by lattice mismatch and thermal expansion coefficient differences between the Si substrate 11 and the GaN semiconductor layer 19. The construction of the low temperature AlN buffer layer 13 and the method for manufacturing the same are known in the art. Instead of the low temperature AlN buffer layer 13, super-lattice layers can be formed by alternately stacking AlN layers and GaN layers. The super-lattice structure can prevent, to some degree, crystal defects, such as dislocations, from propagating.
However, even though the buffer layer 13 is grown on the Si substrate 11, the GaN semiconductor layer grown on the buffer layer 13 has many defects due to limitations of physical properties thereof. Accordingly, it is necessary to further enhance the crystallinity of the GaN layer heterogeneously grown on a substrate of for example Si.